Method and radio station for data transmission in a radio communication system

ABSTRACT

A data packet is sent from a first radio station to several second radio stations in a transmission channel, whereby the data packet is provided with a first error protection for a first transmission over the radio interface. Subsequently, the data packet is sent by the first radio station to the second radio station at least one further time, whereby the data packet is provided with a second error protection for further transmission over the radio interface, which has a higher redundancy than the first error protection.

The invention relates to a method and a radio station for datatransmission in a radio communication system, in particular in a mobileradio system.

Radio communication systems are known in the form of mobile radiosystems and wireless user access networks, also called wireless localarea networks (WLANs). Mobile radio systems according to the prior artare, for example, the second-generation worldwide GSM system (GlobalSystem for Mobile Communications) and the future third-generation UMTSsystem (Universal Mobile Telecommunications System). These systemsoperate in frequency bands around approx. 1 and 2 GHz and use a TDMA(Time Division Multiple Access) or CDMA (Code Division Multiple Access)user separation method. Future WLAN systems are, for example, theHiperlan/2 system standardized by ETSI (European TelecommunicationsStandards Institute) and the IEEE 802.11 system standardized by theIEEE. Both systems use an OFDM (Orthogonal Frequency Division Multiplex)transmission method with 64 subcarriers in the 5 GHz frequency band.These two WLAN systems are known for example from the book by B. Walketitled “Mobilfunknetze und ihre Protokolle” (“Mobile radio networks andtheir protocols”), B. G. Teubner, Stuttgart, 2000, Vol. 2, pages370-432.

Both WLAN systems are based on a simple network structure, according towhich elements called “access points” (APs) provide radio-basedhigh-speed access at a typical data rate of 27 Mbit/s to variousnetworks, such as, for example, UMTS core networks, ATM networks andIP-based networks. At the same time a potential mobility of the userstations, known as mobile terminals (MTs), is enabled in that handovermethods, i.e. a connection transfer from one access point to a nextaccess point, are supported.

A fundamental problem in radio systems of this kind arises during thetransmission of distribution services, known as broadcast or multicastservices, since data is often corrupted due to the differentcharacteristics of the radio channels.

According to the prior art, combinations of a forward error correction(FEC) mechanism and ARQ (Automatic Repeat Request) methods are mostlyemployed in radio communication systems for what are termed unicastconnections, i.e. point-to-point connections between two radio stations,in order to guarantee a required quality of service (QoS) or residualerror probability during the transmission. If FEC methods fail in poorradio channel conditions, such as fading for example, or due to stronginterference factors, data either received incorrectly or not receivedat all by the receiver can be restored with a high degree of probabilityby a retransmission according to the ARQ method.

With the ARQ method, correctly received data packets are acknowledged bymeans of positive acknowledgements (ACK=Acknowledgement), and in thecase of incorrectly received data packets a request for retransmissionis sent by the receiver by means of a negative acknowledgement(NAK=Negative Acknowledgement) in a signaling packet data unit (S-PDU).

According to the prior art, exclusive use is made of FEC methods toassure reliable data transmission for distribution services or alsogeneral signaling systems. ARQ methods are not used to that effect sincethe transmission capacities required for this on the return channelswould be too great, and as the number of users increased the probabilityof an error-corrupted data transmission would increase in any case. Tosupplement a strong FEC method, a method known as repetition mode isused for example in the Hiperlan/2 system described. With this mode, thesame data is transmitted a number of times in succession without anacknowledgement of receipt being transmitted on the part of thereceiving radio stations. However, even a multiple transmission of datapackets of this kind still does not guarantee error-free reception.

The object of the invention is therefore to specify a method whichenables reliable transmission of data of a distribution service. Thisobject is achieved by the features of the independent claims.Advantageous developments of the invention can be derived from thedependent claims.

According to the invention, a data packet is provided with a first errorprotection means for a first transmission over the radio interface ofthe radio communication system. In the subsequent at least one furthertransmission the data packet is provided with a second error protectionmeans which has an increased redundancy compared with the first errorprotection means.

Advantageously, an increased probability of the correct reception of thedata packet at the location of the second radio stations is ensuredowing to the increased redundancy for the further transmission, as aresult of which its use for a point-to-multipoint transmission appearsadvantageous. Furthermore the energy consumption of the second radiostation, embodied for example as a mobile radio station, canadvantageously be reduced, since following a successful reception of thefirst transmission of the data packet subsequent transmissions no longerhave to be received and evaluated.

According to a development of the invention, the redundancy is increasedin accordance with an ARQ method with incremental redundancy. Incombination with the development based on this feature, according towhich the reception of the data packet is not acknowledged by the secondradio stations, the known Type 2 ARQ method, for example, is adapted inan optimal manner to the conditions of the data transmission indistribution services. The implementation corresponds in this case to anefficient ARQ method without use of a return channel. At the same timedata packets not received or received corrupted are not requestedexplicitly by means of a negative acknowledgement, but instead negativeacknowledgements are assumed on the transmitter side without saidnegative acknowledgements actually having been received. Thus, as aresult of the ARQ method based on an incremental redundancy the samedata packets are not transmitted a number of times, but different datapackets are transmitted, whereby the information contained therein canalready be extracted from the first data packet. If multiple datapackets are used for decoding the information when transmission errorsare present, then the probability of a correct decoding isadvantageously increased. With the exemplary Type 2 ARQ method,virtually arbitrarily small coding rates can be advantageously definedand consequently more redundancy added to a data packet than would bepossible with the known FEC methods.

According to a further development of the invention, the first and/orsecond error protection means are chosen as a function ofcharacteristics of the radio interface. In this way more or lessredundancy is added to the data packets, for example depending on thecurrent radio channel characteristics, and if necessary more or fewerfurther data packets are transmitted with incremental redundancy. Withgood transmission characteristics, for example, the number of furtherdata packets is reduced, as a result of which the transmission capacityin the transmission channel is advantageously increased. The respectivedifference or deviation between the coding rates can also be set as afunction of the characteristics of the radio interface. The first radiostation or a higher-ranking entity of the radio communication system candetermine the characteristics for example from known signaling schemesof the second radio stations via the radio channel characteristics,request them from the second radio stations using a special signalingscheme or derive them from received signals of the second radiostations.

According to a further development of the invention, the furthertransmission of the data packet with the second error protection meansis executed only after a specific time interval. As a result, comparedwith the repetition mode already described, the data packets aretransmitted not in immediate succession, but in time-staggered fashionso that, for example, the fading effects of the radio channel canadvantageously be exploited. If, for example, when an FEC method isused, a retransmission of the data packet during a fading period wouldnot enable a correct decoding to be performed in certain circumstances,with the use of an incremental redundancy method a successful decodingcan be enabled by transmission of a further, incrementally coded datapacket at a time at which the radio channel exhibits more favorablecharacteristics.

The method according to the invention is used particularlyadvantageously in radio communication systems based on a TDD (TimeDivision Duplex) method. TDD means that the transmission takes placeboth in the uplink and in the downlink direction in a commontransmission frame. Examples of systems of this kind are the Hiperlan/2and IEEE 802.11 systems cited at the beginning.

Further features and advantages of the invention are derived from theclaims and the following description of an exemplary embodiment withreference to the drawing, in which:

FIG. 1 shows a structure of a radio communication system in which themethod according to the invention is performed.

FIG. 1 shows a section of a radio communication system of the typeimplemented, for example, in the Hiperlan/2 system. The structure of thecomponents relevant to the method according to the invention can betransferred in identical fashion to other radio communication systems. Abase station AP (Access Point) is connected via, for example, anInternet protocol-based transmission network IP to further networkcomponents and provides what is called an access point to this networkfor user stations MT (Mobile Terminal) located in the radio coveragearea of the base station AP. The transmission over the radio interfacetakes place here in the downlink direction DL from the base station APto the user stations MT and also in the uplink direction UL from theuser stations MT to the base station AP, and because of the variantcharacteristics of the radio interface is exposed to fault conditionsdue to fading, shadowing, interference, etc.

According to FIG. 1, the base station sends data packets dp to thereceiving user stations MT in a distribution channel BCH (BroadcastChannel). In this case said data packets can contain for examplesignaling and/or user data of services. According to the invention, thebase station AP sends the data packets dp in the distribution channelBCH a number of times in each case, in the process strengthening theerror protection of the respective data packet dp by increasing theredundancy for each further transmission. The correct reception of thedata packets dp is not acknowledged by the receiving user stations MT.Instead, the number of data packets dp sent in addition and theredundancy increment are adjusted by the base station AP to match thecurrent characteristics of the transmission channel.

1-11. Cancel
 12. A method for data transmission in a radio communicationsystem, comprising: sending a data packet by a first radio station to aplurality of second radio stations via a transmission channel in a firsttransmission over a radio interface with a first error protectionmechanism; and subsequently sending the data packet by the first radiostation at least one additional time to the second radio stations overthe radio interface with a second error protection mechanism which hasan increased redundancy compared with the first error protectionmechanism.
 13. The method according to claim 12, wherein the data packetincludes at least one of user data and signaling information.
 14. Themethod according to claim 13, wherein the increased redundancy when thedata packet is subsequently sent at least one additional time iseffected according to an Automatic Repeat Request method withincremental redundancy.
 15. The method according to claim 14, furthercomprising not acknowledging each correct reception of the data packetby the second radio stations.
 16. The method according to claim 15,further comprising at least one of the first and second error protectionmechanisms as a function of characteristics of the radio interface. 17.The method according to claim 16, further comprising choosing a numberof additional times the data packet is subsequently sent as a functionof characteristics of the radio interface.
 18. The method according toclaim 17, wherein said subsequently sending of the data packet with thesecond error protection mechanism is performed by the first radiostation after a specific time interval has elapsed.
 19. The methodaccording to claim 18, wherein the data transmission takes place in theradio communication system in accordance with a wireless local areanetwork standard.
 20. The method according to claim 19, wherein one ofHiperlan/2 and IEEE 802.11 is used as the wireless local area networkstandard.
 21. A radio communication system having a plurality of radiostations, comprising: a radio station sending a data packet to otherradio stations via a transmission channel in a first transmission over aradio interface with a first error protection mechanism and subsequentlysending the data packet at least one additional time to the other radiostations over the radio interface with a second error protectionmechanism which has an increased redundancy compared with the firsterror protection mechanism.
 22. The radio communication system accordingto claim 21, wherein said radio station is a base station.